Data Structures and Algorithms in Java[1].pdf - Fulvio Frisone

An iterator is a software design pattern that abstracts the process of scanning
through a collection of elements one element at a time. An iterator consists of a
sequence S, a current element in S, and a way of stepping to the next element in S
and making it the current element. Thus, an iterator extends the concept of the
position ADT we introduced in Section 6.2. In fact, a position can be thought of as
an iterator that doesn't go anywhere. An iterator encapsulates the concepts of
"place" and "next" in a collection of objects.
We define the iterator ADT as supporting the following two methods:
hasNext(): Test whether there are elements left in the iterator.
next(): Return the next element in the iterator.
Note that the iterator ADT has the notion of the "current" element in a traversal of a
sequence. The first element in an iterator is returned by the first call to the method
next, assuming of course that the iterator contains at least one element.
An iterator provides a unified scheme to access all the elements of a collection of
objects in a way that is independent from the specific organization of the collection.
An iterator for an array list, list, or sequence should return the elements according
to their linear ordering.
Simple Iterators in Java
Java provides an iterator through its java.util.Iterator interface. We
note that the java.util.Scanner class (Section 1.6) implements this
interface. This interface supports an additional (optional) method to remove the
previously returned element from the collection. This functionality (removing
elements through an iterator) is somewhat controversial from an object-oriented
viewpoint, however, and it is not surprising that its implementation by classes is
optional. Incidentally, Java also provides the java.util.Enumeration
interface, which is historically older than the iterator interface and uses names
hasMoreElements() and nextElement().
The Iterable Abstract Data Type
In order to provide a unified generic mechanism for scanning through a data
structure, ADTs storing collections of objects should support the following
method:
iterator(): Return an iterator of the elements in the collection.
This method is supported by the java.util.ArrayList class. In fact, this
method is so important, that there is a whole interface,
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